Transmission equalization



Sept. '1, 1925.

' A. B. CLARK TRANSMISSION EQUALIZATION Filed Jan. 23. 1920 INVENTOR.

ATTORNEYS.

Patented Sept. 1, 1925. q

UNITED STATES PATENT OFFICE.

ALVA B. CLARK, OF BROOKLYN, NEW YORK, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

TRANSMISSION EQUALIZATION.

Application filed January 23, 1920. Serial No. 353.487.

To all whom it may concern:

Be it known that I, ALVA B. CLARK, residing at Brooklyn, in the county of Kings and State of New York, have invented certain Improvements in Transmission Equalization, of which the following is a specification.

This invention relates to transmission circuits and more particularly to arrangements for regulating the electrical characteristics of such circuits. The principal object of the invention is to provide suitable methods and means for maintaining substantially constant the transmission equivalent of a circuit, regardless of variations in the conditions affecting the electrical characteristics of the circuit. Other and further objects and features of the invention will appear more fully from the detailed description of the invention hereinafter given.

In general,-the transmission equivalent of a circuit varies from time to time, due to a number of different causes. Among these causes may be enumerated changes in the re,- sis'tance of the circuit due to changes in temperature, it having been found from compu tations based on an average yearlychange in temperature of approximately 75 F. that there will be considerable change resulting in the resistance of a circuit which will produce variations in the transmission equivalent of such circuit. Accordingly it is highly desirable that some provision be made for overcoming the effect of these variations so that transmission over the circuit may be rendered uniform.

One of the arrangements for eliminating such difficulties consists in varying some adjustable element of the transmission circuit to compensate for the variation introduced into said circuit. More specifically the method of operation of such anarrangement involves compensating for the effects due to some one cause, such as changes in resistance due to temperature variations, by observing the effects produced in one circuit of a cable, then varying in accordance with such changes similar elements of a number of circuits which are affected in a like manner. These results are accomplished by arranging the first mentioned or pilot circuit in one element of a Wheatstone bridge arrangement so that variable currents will flow through the bridge with variations in the conductivity of the pilot clrcult. Apparatus in the bridge circuit is so arranged as to respond to the currents flowing therein, to control an automatic switch for adjusting the potentiometers of the amplifiers in the other transmission circuits. Such an arrangement may operate automatically and will serve as a means for regulating and maintaining the transmission equivalent of a circuit substantially constant.

In the arrangements above described the regulation of the transmission is the same for currents of all frequencies which are transmitted over the circuit. This is usually satisfactory for the types of lines normally used and particularly for currents having frequencies within the voice range. However, such an arrangement may not be satisfactory for use on certaintypes of lines, such, for example, as long four-wire cable circuits of very light loading and over which it is contemplated to transmit relatively low frequency signaling currents in the same manner as the telephonic currents. This is due tothe fact that the variations in transmission with changes in resistance, due to temperature variations, will be different for the telephonic currents than for the currents of the lower frequencies used for signaling purposes, such, for example, as currents of approximately 133 cycles; a given resistance change causing a smaller variation of the 133 cycle loss than of the loss at voice frequencies. Computations which have been made based on a temperature change of approximately 75 F.-, as may readily occur during the course of a year, indicate that the transmission loss at 133 cycles, relative to the loss at voice frequencies, will change approximately 40 miles if the cable circuit is 1000 miles long. In other words, if the transmission loss at voice frequencies is maintained constant at the value corresponding to the average temperature, the transmission loss at 133 cycles will be about 20 miles less than this when the temperature is a maximum and approximately 20 miles greater than this when the temperature is a minimum. In order then that these low frequency signaling currents may be trans mitted over the circuit in the same manner as the voice frequency currents and in order that all of these frequencies may be maintained at the same transmission level, it is necessary, not only that means be provided for regulating the transmission efficiency of the circuit to compensate for changes due to such things as temperature variations, but that means be provided also for equalizing the transmission of the different frequencies and it is a particular feature of this invention to provide an arrangement for accomplishing this result.

The invention may now be more fully understood from the following description when read in connection with the accompanying drawing which constitutes a circuit diagram of an arrangement embodying the principles of this invention.

Referring to the diagram, a four-wire circuit is shown interconnecting stations C and D, said four-wire circuit comprising a pair of transmission lines L, and L for transmitting in opposite directions. These transmission lines may include one-way repeaters at intermediate stations (not shown) and are associated at the terminal stations with terminating two-wire lines L and L through the usual three-winding transformers 1 and 2. The two-wire lines are balanced at the terminal stations by means of artificial lines N and N For the purpose of adjusting the transmission equivalent of the four-wire circuits, amplifiers A and A, are provided, the amplification of said amplifiers being controlled by the setting of potentiomcters P and P The setting of the potentiometers is determined by means of an automatic switch S, having wipers S and S,,. It will be understood that where a number of four-wire circuits are subject to the same conditions as for instance, where the conductors of the circuits are included in a common cable, the potentiomcters for the amplifiers of a plurality of such circuits may all be controlled by the common switch S. In order to determine the operation of the switch S a pilot circuit L is provided. said circuit being included in the same cable. or subject to the same conditions. as the fourwire circuits or two-wire circuits. as the case may be, which are to be regulated. The pilot circuit L, is included in one branch of a Wheatstone bridge,of which the resistance R which may be equivalent to the average resistance of the pilot circuit L forms a second branch. Resistances R, and R respectively constitute the third and fourth branches. In circuit with the pilot circuit L and the resistance R, is an adjustable resistance R for balancing the bridge, said resistance being controlled by a wiper S carried by the switch S. The wiper S is included in the bridge circuit of the Wheatstone balance, said circuit also including a polarized relay 3, which controls the operation of th switch S through the medium of the stepping magnets 4 and 5, and the holding magnets 6 and 7. For the purpose of equalizing the transmission of the circuit with respect to the relatively low frequency signaling currents, the inductances B and B are bridged across the sides of the fourwire circuit. In order to avoid impedance reactance which might complicate matters, these inductances have been bridged across the sides of the circuit between the amplifiers A and A and A, and A Variable resistances R and R are also connected in series with these variable inductances, which may be varied initially until the inductance produces the required result. The inductances B and B are controlled by the wipers T and T, of the automaticswitch S.

Assuming that a change takes place in the resistance of the pilot circuit L due to a change in temperature or other cause, the lVheatstone bridge is unbalanced, and a current flows over the bridge circuit through the winding of the polarized relay 3 and over the wiper S If the resistance of the circuit L is increased, this current is in such a direction as to throw the armature of the polarized relay 3 to the left, as shown in the drawing, thereby completing a circuit from battery 9, through the winding and over the back contact of buzzer relay 10, over the left hand contact of polarized relay 3, winding of stepping magnet 4 and winding of release magnet 7. The release magnet 7 draws the pawl carried by the armature of stepping magnet 5 from engagement with the corresponding ratchet wheel, and the stepping magnet 4 operates its ratchet wheel step by step with each interruption of the circuit by the buzzer relay 10 to rotate the wiper S in counter-clockwise direction. The division oi the resistance R between the arms of the bridge, including the pilot circuit L and resistance R is thereby changed so that the VVheatstone bridge is again balanced. This action moves the wipers S and S of the switch S in a counter-clockwise direction, thereby altering the setting of the potentiometers so that the amplification of amplifiers A and A, is increased. If the change in the con dition of the pilot circuit L is of such na ture as to cause a reduction of the resistance, the current fiows through the windings of the polarized relay 3 in the opposite direction and a circuit is closed through the buzzer relay 10, stepping magnet 5 and release magnet 6, to step the switch S in a clockwise direction thereby decreasing the amplification of the amplifiers A and A By providing the switch S with a suflicient number of wipers such at S and S the transmission equivalent of as many circuits as desired may be regulated, assuming, of course, that similar variations take place in all of the circuits.-

As a result of the above described operation of switch S, the transmission loss for the voice frequency currents is thus adjusted to the required value. However, as has been pointed out, the transmission loss at 133 cycles will then be different fromthat at the voice frequencies, since the 133 cycle equivalent has not been affected by the tem- T and T,. which adjust the inductance B and B to different values, will also'be r0? tated in a counter-clockwise direction, while the above operation involving ,the'potenti ometers P and P has been taking place;

The counterclockwise rotationof'the wipers T and T causes the inductances B3 and B to have lower values, so that the transmission loss which they introduce, due to being bridged across the circuit, 1s increased. It

is pointed out that thi's'lowering of the in ductance value will cause a; larger cll'ect for low frequency currents than for l'i'igh frequency currents. VVith this arrangement the transmission loss for the 133 cycle signaling current can be made approximately equal to the loss at voice frequencies. The converse of this operation is also true so that when the resistance of circuit'5 decreases and causes wipers S and S to rotate in a clockwise direction to increase the amplification of the potentiometers, tl1ewipers '1 and T, will also be rotated in a clock.- wise manner to cause the inductances B and B to have greater values, so that the transmission loss which they introduce, due to being bridged across the circuit is decreased. This increasing of the inductance value will cause a smaller effect for low frequency currents than for high frequency currents, and accordingly the transmission loss for the 133 cycle signaling currents may again be made approximately equal to the loss at voice frequencies. It may furthermore be advisable to insert resistances between each contact point on which the wipers T and T bear, and the taps on the inductances B and B,. By doing this the time constants of these inductance coils can be kept constant at all settings, or these may be made different for each setting, if this is found necessary in order to secure proper equaliza-' tion.

It will be seen that by means of the above described arrangements variations in conditions which affect a number of circuits alike may be substantially compensated for and, furthermore, that such an arrangement will be operative with currents of different frequencies which may be transmitted over the circuits, While specific reference has been made to signaling currents of approximately 133 cycles frequency, it is understood that the compensation system will apply to all frequencies for which the transmission changes due to the temperature variations may be of different amounts.- Accordingly it will be obvious that the general principles herein=disclosed-may be embodied in'many other""organij7ations widely different from that 7 illustrated, without departing from the spirit of" ie' i n vention as defined in the following cla ms. L

What is claimed is: v,

1.'In a signalingsystem, aplurality of transmission circuits, amplifying means in said circuits, variable inductances bridged across said circuits,a pilot'circuit subject to influences'similar to those affecting the transmission efliciency of said transmission circuits, an automatic switch, means to set said switch in accordance with theelectrical conditions of said pilot circuit, and means controlled by thesetting o'fs'aid switch for adjusting the amplification of said amplifying means and for varyingsaid inductances.

'2. In a signaling system, a transmission line in which the attenuation caused by changes in the electrical characteristics of said circuit 'var'iesto a greater degree with respect 'to "currents' 'ofi certain frequencies than with respect tocurrents of 'other frequencies, and means associated'with said circuit and"respondii1gwhen the electrical characteristics of'said circuit change for equalizing the attenuation at the different frequencies.

.3. In a signaling system, a transmission circuit in which the transmission equivalent varies upon changes vin the electrical characteristics of said circuit to a greater degree with respect to currents of certain frequenciesthan with respect to currents of other frequencies, means associated with said circuit and responding when the electrical characteristics of said circuit change for causing the transmission equivalent of said circuit to vary to the same degree with respect to currents of both of said frequencies, and means associated with said circuit and responding when the electrical characteristics of said circuit change for compensating for the changes in the transmission equiva lent caused by said change in electrical characteristics.

4. In a signaling system, a plurality of transmission circuits, vacuum bulb amplifiers in said circuits. variable inductancesincluded in circuits bridged across said circuits between said amplifiers, a pilot circuit subject to influences similar to those affect- .ing the transmission efficiency of said transplifier having input terminals, means whereby currents transmitted over said line produce an electromotive force at said terminals, a regulating device adjustable to different settingsfor causing changes in the electromotive force impressed on said terminals proportionate to changes in the transmission characteristics of the line, said device comprising means for causing said change in electromotive force to vary with frequency for a given change in setting.

6. The combination with a line whose transmission characteristics vary of a repeater comprising a regulating device adjustable to different settings for causing changes in the gain of said repeater proportionate to changes in the transmission characteristics of the line, said device omprising means for causing said changes of repeater gain to vary with frequency for a given change in setting.

7. The combination with a line whose transmission characteristics vary of a repeater comprising a regulating device capable of adjustment to different settings for causing changes in the gain of said repeater proportionate to changes in the transmission characteristics of the line, said device comprising means for causing said changes to be smaller for low frequencies than for high frequencies for. a given ehaiige of setting of said device.

8. The combination with a line whose transmission characteristics vary of a repeater comprising means capable of adjustment to various settings for causing sub stantial changes of repeater gain both for signaling frequencies of the order of 135 cycles per second and for the upper frequencies of the voice range, said means comprising means for causing said changes to be smaller for the low frequencies than for the high frequencies for a given change in setting of said lirst mentioned means.

9. The combination with a line whose transmission characteristics vary of an amplifier con'iprising two amplifying elements in tandem relation and means connected between said elements for varying the degree by amounts varying with frequency and inproportion to changes in the transmission characteristics of said line.

10. The combination with a line whose transmission characteristics vary of a translating device having input terminals, means whereby currents of different frequencies transmitted over the line produce electromotivc forces of diflering frequencies at said terminals, means associated with said device for passing high frequencies more readily than low frequencies, and voltage reducing means connected between said source and said terminals for producing voltage changes proportionate to changes in the transmission characteristics of the line, said voltage reducing means comprising means for producing a greater reduction in the high frequency voltage supplied to said terminals than in the low frequency voltage supplied to said terminals.

11. The combination with a line subject to changes in its transmission equivalent which vary with frequency over the range up to the highest important voice frequency, of a regulating device and operating means therefor for so controlling said line as to compensate for said changes over the range of frequencies from signaling frequencies of the order of 13:) cycles per second to the highest lllll,)(ll'tt1l1t voice frequencies.

12. The combination with a line subject to changes of its transmission equivalent with temperature which vary with frequency, of a repeater therefor, comprising a regulating device for causing repeater gain changes to so vary with frequency for each change in the setting of said device as to compensate for the variations in said changes in the transmission equivalent over a range of frequencies from signaling frequencies of the order of 135 cycles per second to the highest important voice frequencies, and means including a pilot wire subject to the temperature changes to which said line is subject for automaticaly setting said device in response to saidtemperature changes.

In testimony whereof, I, have signed my name to this specification this 22nd day of January 1920.

ALVA B. CLARK. 

